In the case of patients with neurological or psychiatric illnesses, for example Parkinson's disease, essential tremor, dystonia or compulsion disorders, nerve cell groups in defined areas of the brain, for example the thalamus and the basal ganglia are active because of the illness, for example being excessively synchronous. In this case, a large number of neurons form synchronous action potentials; the neurons involved fire excessively synchronously. In healthy persons, in contrast, the neurons fire qualitatively differently in these brain regions, for example in an uncorrelated manner.
In the case of Parkinson's disease, the pathologically synchronous activity, for example of the thalamus and of the basal ganglia, changes the neural activity in other brain regions, for example in areas of the cerebral cortex, such as the primary motor cortex. In this case, the pathologically synchronous activity in the area of the thalamus and of the basal ganglia forces itself, for example, on the rhythm of the cerebral cortex so that, in the end, the muscles which are controlled by these areas carry out a pathological activity, for example a rhythmic tremor.
In the case of patients who cannot (can no longer) be treated with medicaments, a deep electrode is implanted on one side or both sides depending on the clinical signs and depending on whether the illness occurs on one side or both sides. In this case, a cable leads under the skin from the head to the so-called generator, which comprises a controller with a battery and, for example, is implanted under the skin in the area of the clavicle. The deep electrodes are used to carry out a continuous stimulus with a high-frequency periodic sequence (pulse train at a frequency of >100 Hz) of individual pulses, for example square-wave pulses. The aim of this method is to suppress the firing of the neurons in the target regions. The effective mechanism on which the standard deep stimulation is based has not yet been adequately explained. The results of a number of studies indicate that standard deep stimulation acts in the same way as reversible lesion formation, that is to say in the same way as reversible deactivation of the tissue: standard deep stimulation suppresses the firing of the neurons in the target regions and/or in brain areas linked thereto.
This type of stimulation has the disadvantage that the power consumption of the generator is very high, so that the generator including the battery frequently has to be replaced operatively after only about one to three years. It is even more disadvantageous that the high-frequency continuous stimulation, as a non-physiological (unnatural) input in the area of the brain, for example of the thalamus or of the basal ganglia, can lead to adaptation of the relevant nerve cell groups over the course of a few years. In order to achieve the same stimulation success, a greater stimulus amplitude must then be used for stimulation, as a consequence of this adaptation. The greater the stimulus amplitude is, the greater is the probability that the stimulation of adjacent areas will lead to side-effects—such as dysarthria (speech disturbances), dysesthesia (in some cases highly painful false sensations), cerebellary ataxia (incapability to stand safely without an external aid), or symptoms similar to schizophrenia, etc. These side-effects cannot be tolerated by the patient. In these situations, the treatment thus loses its effectiveness after a few years.
In the case of other stimulation methods, such as those described in DE 102 11 766.7 A1, it has been proposed that stimuli be applied in the respective target region on a demand-controlled basis. The aim of these methods/these apparatuses is not simply to suppress the illness-synchronous firing—as in the case of standard deep stimulation—but to change this to be closer to the physiological, uncorrelated firing pattern. The aims of this are on the one hand to reduce the power consumption and on the other hand to use the demand-controlled stimulation to reduce the amount of energy introduced into the tissue in comparison to the standard deep stimulation.